通过多列在两个实体之间创建多对多关系

Question

I have the following four tables:

Customer

Id
FirstName
...

ConsumptionPoint

Id
Address
...

Invoice

Id
InvoiceNumber
CustomerId
ConsumptionPointId
...

ContractAccount

Id
ContractAccountNumber
CustomerId
ConsumptionPointId
IsCurrentDelivery
...

I want to get the ContractAccountNumber for an Invoice.

Is it possible to create some kind of a relation between these both to access the ContractAccount(s) of an Invoice directly?

Currently I'm doing something like:

invoice.Customer.ContractAccounts
    .Where(ca => ca.ConsumptionPoint == invoice.ConsumptionPoint &&
    ca.IsCurrentDelivery == true).FirstOrDefault();

update in SQL I would simply do a join with multiple conditions:

SELECT i.Id AS InvoiceId, ca.Id AS ContractAccountId, 
ca.ContractAccountNumber
FROM Invoices i
LEFT JOIN ContractAccounts ca
ON i.ConsumptionPointId = ca.ConsumptionPointId
AND i.CustomerId = ca.CustomerId
WHERE ca.IsCurrentDelivery = 1

update 2:

Basically I just want to get rid of the ca.ConsumptionPoint == invoice.ConsumptionPoint in the Where-Clause and want to define this inside the relation.

Actually this is a many-to-many relationship: one Invoice can link to many ContractAccounts (via different Customer/ConsumptionPoint combinations) and one ContractAccount can link to many Invoices. Is there no way to tell .net to build a many-to-many relationship, based on the combination of two custom columns?

Answer 1

I do not think that you need two keys for that. Using EF core you can create a relationship like that:

Invoice:

public class Invoice
{
  public int Id {get; set;}
  public int InvoiceNumber {get; set;}
  public int CustomerId {get; set;}
  public Customer Customer {get; set;}
  public int ConsumptionPointId {get; set;}

  public int ContractAccountId {get; set;}
  public ContractAccount ContractAccount {get; set;}
}

public class ContractAccount
{
  public int Id {get; set;}
  public int ContractAccountNumber {get; set;}
  public bool IsCurrentDelivery {get; set;}
}

Then configuration:

modelBuilder.Entity<Invoice>()
  .HasOne(b => b.ContractAccount)
  .WithOne()
  .HasForeignKey<Invoice>(b => b.ContractAccountId);

//probably already exists in your code
modelBuilder.Entity<Invoice>()
  .HasOne(b => b.Customer )
  .WithMany()
  .HasForeignKey(b => b.CustomerId);

And then you can access it directly from invoice: invoice.ContractAccount

Answer 2

After some deeper investigation I have found a solution.

Basically it's just a many-to-many relationship. So we can add the appropriate properties to Invoice and ContractAccount Model:

public class Invoice
{
    [Key]
    public long Id { get; set; }
    public Customer Customer { get; set; }
    public ConsumptionPoint ConsumptionPoint { get; set; }
    public virtual ICollection<ContractAccount> ContractAccounts { get; set; }
}

public class ContractAccount
{
    [Key]
    public long Id { get; set; }
    public Customer Customer { get; set; }
    public ConsumptionPoint ConsumptionPoint { get; set; }
    public ICollection<Invoice> Invoices { get; set; }
}

Now, we just have to configure the relationship manually:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);
    modelBuilder.Entity<Invoice>()
        .HasMany(i => i.ContractAccounts)
        .WithOne()
        .HasForeignKey(ca => new { ca.CustomerId, ca.ConsumptionPointId })
        .HasPrincipalKey(i => new { i.CustomerId, i.ConsumptionPointId });
    modelBuilder.Entity<ContractAccount>()
        .HasMany(ca => ca.Invoices)
        .WithOne()
        .HasForeignKey(i => new { i.CustomerId, i.ConsumptionPointId })
        .HasPrincipalKey(ca => new { ca.CustomerId, ca.ConsumptionPointId });
}

And that's it. Now I can do something like:

invoice.ContractAccounts
    .Where(ca => ca.IsCurrentDelivery == true).FirstOrDefault();

which is way better than before.

Thanks for your comments, which pointed me to the right direction.